Your first thought should be "How do I search for this?" A good way to start is put "20 watt audio amplifier discrete components" into your favorite search engine. I did this through Yahoo! and found quite a few to choose from. Further refinement may be done adding in the transistor types separated by semicolons. On to the meat of the question. The 2n3904/3906 transistors are wonderful low powered units suitable for decent (but not the esoteric super audio hifidelity quasi-snob gear) preamplification and up to around ~1 watt of audio (1/2 watt is much easier with these). They will be fine in the front end of the amp. The 2n3055/mj2955 are great for the final stage and have been used in countless amps since their introduction. What is missing from this equation is the driver transistor - the ones used between the preamp (2n3904/3906) and the power section (2n3055/mj2955). This will need to be a medium power transistor type capable of over 1/2 watt of power or greater all on its own (collector current is key here). G(eneral) P(urpose) transistors that are available in the better Radio Shack (or equivalent) stores are usually the 2n2222, 2n4401 or the TIP series(31/32, etc). With that out of the way, a good place to look is ESP's website. Click the Projects arrow and Projects by Category arrow and then Power Amps and Accessories link. Check out projects 03, 03A, 03B, 10, and 12A. 10 looks to be the closest to your specs, with a need to substitute your transistors in for the appropriate ones in the schematic. The key to your power output is to have +-20V power supply, or 40V positive supply. Also, check out this design by Sergio Garcia de Alba Garcin, but you neeed to read the description below the schematic. It could use a bit of tweaking to get up to 20 watts, but it seems close. I have my own personal design using the '04/'06, '01 and '55 transistors that I have tweaked to get many different wattages from, and 20 watts can be had from the previously mentioned +- 20 V supply. But this is on hold from being published until I can get my write up done. Let the community know what you have been able to do / have done. I know we'd really like to see the finished result. Qa
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I have seen many amps with higher output power without using middle stage driver transistors. Is it necessary to use them?
The answer to that is, "it depends". If your output transistors need a healthy amount of current to operate efficiently, or at all, then yes you do. Certain BJT designs require them. Others do not. One thing is for sure, if your amp runs out of steam at the higher powered end of the dial or starts the break up early, then you can suspect the middle drivers if they are the same small signal low power transistors as the preamp. And, if they run very hot to the touch, you are risking failure of that part. All I can tell you is give it a try. If it works to your satisfaction, then all is good. If not, just a couple of bucks will get you some higher powered transistors to set it right. Qa